Rocky Mountains
s with distinct geological origins. Collectively these make up the , a mountain system that stretches from Northern British Columbia through central and which is part of the great mountain system known as the .}} activity that resulted in much of the rugged landscape of the western . The , about 80–55 million years ago, was the last of the three episodes and was responsible for raising the Rocky Mountains. Subsequent erosion by glaciers has created the current form of the mountains.}} Ancestral rock The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces. The oldest rock is that forms the core of the North American continent. There is also Precambrian sedimentary , dating back to 1.7 billion years ago. During the , western North America lay underneath a shallow sea, which deposited many kilometers of and . In the southern Rocky Mountains, near present-day and , these ancestral rocks were disturbed by mountain building approximately 300 Ma, during the . This mountain building produced the Ancestral Rocky Mountains. The uplift created two large mountainous islands, known to s as and , located roughly in the current locations of the and the . They consisted largely of , forced upward through layers of the laid down in the shallow sea. The mountains eroded throughout the late Paleozoic and early , leaving extensive deposits of . Mesozoic deposition in the Rockies occurred in a mix of marine, transitional, and al as local relative sea levels changed. By the close of the , 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognized s. The most extensive non-marine formations were deposited in the period when the western part of the covered the region. Terranes and subduction started to collide with the western edge of North America in the (approximately 350 million years ago), causing the . During the last half of the , much of today's , , , and were added to . Western North America suffered the effects of repeated collision as the and s sank beneath the continental edge. Slivers of continental crust, carried along by subducting ocean plates, were swept into the subduction zone and scraped onto North America's western edge. These terranes represent a variety of tectonic environments. Some are ancient island arcs, similar to Japan, Indonesia and the Aleutians; others are fragments of oceanic crust obducted onto the while others represent small isolated mid-oceanic islands. generated above the subducting slab rose into the North American continental crust about inland. Great arc-shaped volcanic mountain ranges, known as the , grew as and spewed out of dozens of individual es. Beneath the surface, great masses of were injected and hardened in place. For 270 million years, the effects of plate collisions were focused very near the edge of the boundary, far to the west of the Rocky Mountain region. It was not until 80 that these effects began to reach the Rockies. Raising the Rockies from between 80 and 55 Ma. For the Canadian Rockies, the mountain building is analogous to a rug being pushed on a hardwood floor: the rug bunches up and forms wrinkles (mountains). In Canada, the subduction of the and the terranes smashing into the continent are the feet pushing the rug, the ancestral rocks are the rug, and the in the middle of the continent is the hardwood floor. }} inland from a boundary. Geologists continue to gather evidence to explain the rise of the Rockies so much farther inland; the answer most likely lies with the unusual subduction of the , or possibly due to the subduction of an .}} At a typical subduction zone, an oceanic plate typically sinks at a fairly steep angle, and a grows above the subducting plate. During the growth of the Rocky Mountains, the angle of the subducting plate may have been , moving the focus of melting and mountain building much farther inland than is normally expected. It is postulated that the shallow angle of the subducting plate greatly increased the friction and other interactions with the thick continental mass above it. Tremendous piled sheets of crust on top of each other, building the extraordinarily broad, high Rocky Mountain range. The current southern Rockies were forced upwards through the layers of and sedimentary remnants of the Ancestral Rocky Mountains. Such sedimentary remnants were often tilted at steep angles along the flanks of the modern range; they are now visible in many places throughout the Rockies, and are prominently shown along the , an early Cretaceous sandstone formation that runs along the eastern flank of the modern Rockies. Current landscape : a high plateau, probably above sea level. In the last 60 million years, stripped away the high rocks, revealing the ancestral rocks beneath, and forming the current landscape of the Rockies.}} as shown here, have dramatically shaped the Rocky Mountains.}} Periods of glaciation occurred from the Epoch (1.8 million–70,000 years ago) to the Epoch (fewer than 11,000 years ago). The s left their mark on the Rockies, forming extensive landforms, such as U-shaped valleys and s. Recent glacial episodes included the that began about 150,000 years ago and the that probably remained at full glaciation until 15,000–20,000 years ago. Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation. The was a period of glacial advance that lasted a few centuries from about 1550 to 1860. For example, the and s in reached their most forward positions about 1860 during the . All of the geological processes, above, have left a complex set of rocks exposed at the surface. For example, in the Rockies of Colorado, there is extensive and dating back to the Ancestral Rockies. In the central Canadian Rockies, the main ranges are composed of the Precambrian s, while the front ranges are composed of the limestones and dolomites. Volcanic rock from the (66 million–1.8 million years ago) occurs in the and in other areas. Millennia of severe erosion in the transformed intermountain basins into a relatively flat terrain. The and other north-central ranges contain folded and faulted rocks of and age draped above cores of and igneous and metamorphic rocks ranging in age from 1.2 billion (e.g., Tetons) to more than 3.3 billion years ( ). Elevation map References Category:Earth